Filter or separation methods are of great economic importance for the concentration and separation of gases and liquids. In this field, membrane technology is used to a constantly increasing extent. In this instance, the use of hollow fiber membranes in a fiber or concentration process has a number of advantages.
Since the relationship between surface and volume is relatively high, the efficiency of the concentration or fiber process will also be high; moreover, the method is easily carried out and adapted to the respective requirements.
Previous commercially common hollow fiber membranes were made out of polymer material and, as a result, have been sensitive to corrosive conditions and higher temperatures. For this reason, soon ceramic hollow fiber membranes were manufactured by condensing steam on a carbon wire and by subsequently removing the wire.
The methods for the manufacture of ceramic hollow fibers known today include a spinning process in which, in a first step, elastic green fibers are manufactured out of a spinnable mass including precursors of the ceramic material and out of polymer. Subsequently, the polymer content is burned at high temperatures and pure ceramic hollow fibers result.
Owing to the structural properties and their geometry, the ceramic hollow fiber membranes reach, on the basis of metal alloys, for example, Al2O3, only low strength values. For this reason, for example, the breaking force in the 3-point bending test only lies at approximately 6 N, which, when converted, equates to a flexural strength of approximately 33 MPa.
High mechanical stresses of the hollow fiber membranes occur particularly during handling and further processing (for example, in modular construction) and when transporting the hollow fibers (as individual elements or in the form of a module) and during operation at pressure surges or pressure gradients so that, in this instance, partially high reject rates are recorded. Particularly complex is the removal of defects, for example, fiber fractures during transport when the hollow fibers are already installed in a module.
In this regard, the publication DE 10 2009 038 814 A1 discloses a modular element, including a high temperature stable base body having at least one continuous opening for inserting a ceramic capillary membrane, and the publication EP 0 941 759 A1 describes a method for the manufacture of a hollow fiber membrane module.
It is known from the publication CN 102515817 A to manufacture ceramic hollow fiber membranes having a high filter flow rate. In this instance, the composition of these fibers are (in wt %) 40-80 parts powdered Aluminum silicate hydrate, 10-20 parts powdered soft refractory clay, 1-10 parts powdered albite, 1-10 parts powdered dolomite, 6-10 parts powdered borosilicate glass powder, 1-7 parts carboxymethyl cellulose and 1-3 parts glycerol. The outer diameter of these fibers is 3-6 mm, the inner diameter is 1-2 mm and the length of the fibers is 100-1000 mm. The total porosity is 50-60%, the flow rate of water is 10,000-12,500 l/m2 hMPa and the flexural strength is at 35-85 MPa.